The interest in photovoltaic (“PV”) cells in both terrestrial and non-terrestrial applications continues as concerns over pollution and limited resources continue. Irrespective of the application, and as with any energy generation system, efforts have been ongoing to increase the output and/or increase the efficiency of PV cells. In terms of output, multiple cells or layers having different energy bandgaps have been stacked so that each cell or layer can absorb a different part of the wide energy distribution in the sunlight.
The prior art consists of photovoltaic cells with window layers that are nominally lattice-matched to the cell layers beneath them. The constraint of lattice matching fixes the value of the indirect and direct bandgaps of window layers composed of ternary semiconductors. Light with photon energy greater than that of the direct bandgap of the window material will be strongly absorbed in the window layer. Minority-carrier lifetimes and diffusion lengths are often low in many window materials, so that it is preferable that the window is highly transmissive, allowing light to reach the cell emitter and/or base layers beneath the window, where photogenerated carriers can diffuse to the collecting junction more easily before recombining. Therefore, relatively low bandgaps available in lattice-matched window materials are a disadvantage, since they lead to strong absorption of light in the window where it is not used efficiently.